This commit removes a large amount of code partially duplicated
between the text and the graphics windows, and opens the path to
having more than one model window on screen at any given time,
as well as simplifies platform work.
This commit also adds complete support for High-DPI device pixel
ratio. It adds support for font scale factor (a fractional factor
on top of integral device pixel ratio) on the platform side, but not
on the application side.
This commit also adds error checking to all Windows API calls
(within the abstracted code) and fixes a significant number of
misuses and non-future-proof uses of Windows API.
This commit also makes uses of Windows API idiomatic, e.g. using
the built-in vertical scroll bar, native tooltips, control
subclassing instead of hooks in the global dispatch loop, and so on.
It reinstates tooltip support and removes menu-related hacks.
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